Pretty good blog post at liquidmatrix on SCADA
It is a bit condesending but all the facts seem straight.
One item in contrast. He mentions that very few Connections are via IP. That is certainly true but the number is getting larger. Also despite the fact that most connections are via RS 232 or other mechanisms there is usually an interface system now that is almost always IP connected. Often this is the historian but there are others as well. Rockwells RSLinx is an example. Most vendors have them.